The invention relates to a novel system for the recording of data by the storage of digital information in optic form.
In systems for the optic storage of digital information, the maximum possible storage density is limited by diffraction at the wavelength of the writing or reading operations. For the optic wavelengths commercially available in the form of laser diodes, this limit on density can be placed, for example, in the region of one bit per square micrometer. Beyond this limit, the laser light is diffracted by the molecular structure of the material in which the recording is etched, and it is no longer possible to distinguish adjacent elementary dots.
To increase the density of digital information that can be stored in a disk, i.e. to increase the quantity of information stored without increasing the surface area of the disk or to reduce the surface area of the disk without reducing the quantity of information, the use of a stacking of photosensitive layers reacting with different wavelengths (like superimposed photographic layers) has already been proposed: information elements are stored in each layer and each of them can be read only by the specific laser having a well-determined wavelength that is different from the wavelengths used to read the information stored in the other layers. The absorption peaks of the different layers should be at wavelengths that are clearly separated from one another to enable discrimination among the pieces of information. The upper layers have to be transparent to the wavelengths of sensitivity of the lower layers. The difficulty therefore lies in finding, for the different layers, those materials that have the requisite properties in terms of spectral selectivity, sensitivity (for the writing), transparency and information-retaining time.